The Mohorovicic discontinuity, sometimes referred to as "Moho," is the boundary where Earth's crust meets Earth's upper mantle (approximately 31 mi/50 km below the surface), and where seismic waves travel at a different and more rapid rate than the crust or mantle. The Moho is named after Andrija Mohorovicic (1857–1936), a Croatian meteorologist and seismologist who was fascinated with the faults and movements in the earth's infrastructure that result in earthquakes. The discovery of the Moho was most important because it helped scientists discover a second layer, or mantle, inside the earth. It also helped scientists to determine more accurately where this second layer was located in relation to Earth's surface, or crust.

Since the early 1900s, scientists were almost certain that Earth, like an onion, was made up of many layers, but they did not know exactly where the layers started and ended. In 1906, Mohorovicic...